This application claims priority under 35 U.S.C xc2xa7119 from and incorporates by reference the subject matter of European Patent Application No. 01810298.8 filed Mar. 23, 2001 and European Patent Application No. 01203922.8 filed Oct. 16, 2001.
The invention concerns a device with a magnetic position encoder.
Today, contactless position encoders for distances in the sub-millimetre range are used in large numbers in devices for the engineering and automobile industries.
In applications with which the position encoders are exposed to dirt such as dust, fibres, oil gases, etc, magnetic position encoders present a robust and economic alternative to the widely used optical position encoders because they do not require a sealed enclosure between the moving and stationary machine parts of the device.
A device with a magnetic position encoder is known from DE 197 12 829. This device has a piston with a ring-shaped magnet which is movable along an axis. For determining the position of the piston, two magnetic field sensors are foreseen one of which measures the component of the magnetic field produced by the magnet in axial direction and the other one measures the component of the magnetic field produced by the magnet in radial direction. Here, the axial direction of the magnetic field corresponds to the movement direction of the piston. Because the output signals of the two magnetic field sensors are non-linear, the device is not suitable for determining a position signal corresponding to the continuous position of the piston but only for determining whether the piston has reached a predefined position.
A similar device is known from EP 1 074 815. Here, it is suggested that the output signals of the two magnetic field sensors are combined in order to get a position signal which is suitable for determining the continuous position of the piston. The disadvantage is that this position signal is also non-linear.
A further device for determining whether an element has reached a predetermined positioned is known from EP 726 448.
The object of the invention is to develop a device with a magnetic position encoder which delivers a linear signal over a comparatively large working range.
The magnetic position encoder naturally comprises a magnetic field source and a magnetic field sensor which are movable relative to each other along a given path. The magnetic field sensor measures two components of the magnetic field produced by the magnetic field source. A position signal is then derived from the measured components which represents the relative position of magnetic field sensor and magnetic field source. Designs of the position encoder in accordance with the invention are characterised in that the determination of the position signal includes a division of the two measured components of the magnetic field. These designs in accordance with the invention demonstrate the advantage that the position signal is a linear function of the location.
With a preferred embodiment with which the magnetic field source and the magnetic field sensor move relative to each other along a straight line, the magnetic field produced by the magnetic field source is rotationally symmetric in relation to an axis of symmetry. In accordance with the invention, the magnetic field sensor measures two components of the magnetic field both of which lie in a plane running orthogonally to the axis of symmetry of the magnetic field source. With the relative movement of the magnetic field source and the magnetic field sensor, the direction of the magnetic field in this plane changes in relation to the measurement directions defined through the magnetic field sensor. From this change in direction, a linear position signal can be obtained by means of a division of the two measured magnetic field components.
Even when the magnetic field sensor is arranged rigidly and the magnetic field source moves on a circular path, an almost linear position signal can be obtained from the quotient of the two magnetic field components measured in a specific working range.